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.Dd January 28, 2019
.Dt PFIL 9
.Os
.Sh NAME
.Nm pfil ,
.Nm pfil_head_register ,
.Nm pfil_head_unregister ,
.Nm pfil_link ,
.Nm pfil_run_hooks
.Nd packet filter interface
.Sh SYNOPSIS
.In sys/param.h
.In sys/mbuf.h
.In net/pfil.h
.Ft pfil_head_t
.Fn pfil_head_register "struct pfil_head_args *args"
.Ft void
.Fn pfil_head_unregister "struct pfil_head_t *head"
.Ft pfil_hook_t
.Fn pfil_add_hook "struct pfil_hook_args *"
.Ft void
.Fn pfil_remove_hook "pfil_hook_t"
.Ft int
.Fn pfil_link "struct pfil_link_args *args"
.Ft int
.Fn pfil_run_hooks "phil_head_t *" "pfil_packet_t" "struct ifnet *" "int" "struct inpcb *"
.Sh DESCRIPTION
The
.Nm
framework allows for a specified function or a list of functions
to be invoked for every incoming or outgoing packet for a particular
network I/O stream.
These hooks may be used to implement a firewall or perform packet
transformations.
.Pp
Packet filtering points, for historical reasons named
.Em heads ,
are registered with
.Fn pfil_head_register .
The function is supplied with special versioned
.Vt struct pfil_head_args
structure that specifies type and features of the head as well as
human readable name.
If the filtering point to be ever destroyed, the subsystem that
created it must unregister it with call to
.Fn pfil_head_unregister .
.Pp
Packet filtering systems may register arbitrary number of filters,
for historical reasons named
.Em hooks .
To register a new hook
.Fn pfil_add_hook
with special versioned
.Vt struct pfil_hook_args
structure is called.
The structure specifies type and features of the hook, pointer to
the actual filtering function and user readable name of the filtering
module and ruleset name.
Later hooks can be removed with
.Fn pfil_remove_hook
functions.
.Pp
To connect existing
.Em hook
to an existing
.Em head
function
.Fn pfil_link
shall be used.
The function is supplied with versioned
.Vt struct pfil_link_args
structure that specifies either literal names of hook and head or
pointers to them.
Typically
.Fn pfil_link
is called by filtering modules to autoregister their default ruleset
and default filtering points.
It also serves on the kernel side of
.Xr ioctl 2
when user changes
.Nm
configuration with help of
.Xr pfilctl 8
utility.
.Pp
For every packet traveling through a
.Em head
the latter shall invoke
.Fn pfil_run_hooks .
The function can accept either
.Vt struct mbuf *
pointer or a
.Vt void *
pointer and length.
In case if a hooked filtering module cannot understand
.Vt void *
pointer
.Nm
will provide it with a fake one.
All calls to
.Fn pfil_run_hooks
are performed in network
.Xr epoch 9 .
.Sh HEADS (filtering points)
By default kernel creates the following heads:
.Bl -tag -width "ethernet"
.It inet
IPv4 packets.
.It inet6
IPv6 packets.
.It ethernet
Link-layer packets.
.El
.Pp
Default rulesets are automatically linked to these heads to preserve
historical behaviour.
.Sh SEE ALSO
.Xr ipfilter 4 ,
.Xr ipfw 4 ,
.Xr pf 4 ,
.Xr pfilctl 8
.Sh HISTORY
The
.Nm
interface first appeared in
.Nx 1.3 .
The
.Nm
interface was imported into
.Fx 5.2 .
In
.Fx 13.0
the interface was significantly rewritten.
